<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="https://cdn.bootcdn.net/ajax/libs/axios/0.21.1/axios.min.js"></script>
  <script src="https://cdn.bootcdn.net/ajax/libs/qs/6.10.1/qs.min.js"></script>
  <script src="https://cdn.bootcdn.net/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
  <style>
    *{
      margin: 0;padding: 0;box-sizing: border-box;
    }
    ul,li{list-style: none;}

  </style>
</head>
<body>
<style>
html{
  font-size: 16px;
}
.ss1{
  width: 3rem;
  height: 3rem;
  line-height: 3rem;
  font-size: 1rem;
  text-align: center;
  border: 1px solid #000;
}
.ss2{
  width: 5rem;
  height: 5rem;
  line-height: 5rem;
  font-size: 2rem;
  text-align: center;
  border: 1px solid #000;
}
.ss3{
  width: 8rem;
  height: 8rem;
  line-height: 8rem;
  text-align: center;
  font-size: 3rem;
  border: 1px solid #000;
}
</style>

<button id="btn1">减少</button></button>
<button id="btn2">添加</button></button>

<div class="ss1">我们</div>
<div class="ss2">测试</div>
<div class="ss3">发现</div>



<script>
let num = 16;
$("#btn1").click(()=>{
  num-=2;
  $("html").css({
    fontSize:num + 'px'
  })
})

$("#btn2").click(()=>{
  num+=2;
  $("html").css({
    fontSize:num + 'px'
  })
})

</script>

</body>
</html>